Desmoplastic Small Round Cell Tumor (DSRCT) is an embryonal cancer affecting young adults, which is thought to originate in primitive cells of mesothelial origin. The defining histology of DSRCT consists of islets of tumor cells surrounded by reactive fibrosis (desmoplasia), suggesting that tumor/stromal interactions play an important role in the genesis of this cancer. The defining genetic characteristic of DSRCT is a consistent chromosomal rearrangement, fusing the N-terminal transactivational domain (NTD) of the Ewing Sarcoma gene EWS to the three C-terminal zinc fingers of the Wilms tumor suppressor gene WT1, whose DNA binding domain is modulated by alternative splicing (plus/minus insertion of three amino acids, KTS). This complex chimeric transcriptional activator constitutes a single oncogenic gain-of-function mutation that underlies malignant transformation in DSRCT. Identification and characterization of physiologically-relevant target genes of EWS-WT1 will therefore provide important insight into the mechanism of tumorigenesis, and specifically into the interaction between epithelial cancer cells and surrounding stroma, for which DSRCT constitutes a striking model. We have generated osteosarcoma cells with inducible expression of EWS-WT1(plus/minus KTS), and used hybridization to high density microarrays, as well as subtractive PCR, to identify endogenous transcripts whose expression is regulated by EWS-WT1. Extensive validation of these potential targets revealed two novel genes that are directly induced by EWS-WT1 and are expressed in primary DSRCT tumor specimens. We propose to characterize MLF1, a recently identified gene implicated in lineage switching and apoptosis, which is specifically induced by the EWS-WT1(-KTS) isoform, and the novel gene, LRT1, which encodes a novel transmembrane type I glycoprotein that is highly induced by EWS-WT1(+KTS). The mechanism(s) by which these genes are regulated by EWS- WT1(plus/minus kTS), their functional properties and their potential contribution to EWS-WT1-mediated transformation will be studied. Mouse xenograft and cell line models for EWS-WT1- dependent transformation will be established to define the relative contributions of these genes, along with those of two previously characterized EWS-WT1(-KTS) targets, Platelet-Derived Growth Factor-A (PDGF-A) and IL2-receptor beta. Understanding the contribution of target genes regulated by the oncogenic EWS- WT1 chimeric transcription factor will provide important insight into this cancer model, which exemplifies the interactions between tumor cells and their surrounding stroma.